As a conventional speed reduction device, for example, a speed reduction described in the following JP-A-62-4586 is known.
This speed reduction device includes an eccentrically oscillating speed reducer having a plurality of crankshafts axial opposite ends of which are rotatably supported by a carrier and that are spaced apart at equal distances in a circumferential direction, a first external gear that is attached to an output shaft of a motor, and rotates as a driving force is transmitted thereto from the motor, second external gears that are attached to axial one ends of all the crankshafts, and third external gears that are rotatably supported by the output shaft of the motor, and have small-diameter gears that mesh with the second external gears and have a smaller diameter than the second external gears, and large-diameter gears. Here, fourth external gears that are arranged close to axial one ends of all the crankshafts while keeping coaxial relationship with the second external gears, that have large-diameter gears meshing with the first external gear and having a larger diameter than the first external gear, and small-diameter gears meshing with the large-diameter gears of the third external gears and having a smaller diameter than the large-diameter gears is rotatably supported.
However, in such a conventional speed reducer, three-stage external gears, i.e., the second external gears and the large diameter gears and small diameter gears of the fourth external gears are arranged to close one another at an axial one end of each of the crankshafts. Therefore, the whole speed reduction device becomes large in its axial length, and becomes large in its size. Also, gears mesh with each other in three places, that is, between the first external gear and the large-diameter gears of the fourth external gears, between the small-diameter gears of the fourth external gears and the large-diameter gears of the third external gears, and between the small-diameter gears of the third external gears and the second external gears. As a result, there is a problem in that noises become large, and energy loss becomes large. Moreover, since two type of gears (third and fourth external gears) having large-diameter gears and small-diameter gears are used, there is also a problem in that the structure becomes complicated.